1. Field of the Invention
The invention relates to continuously variable transmission systems in which control of the line pressure is effected by both open and closed loop control strategies, the selection being dependent upon the operating conditions of the continuously variable transmission and the clutch. More particularly, the invention relates to a line pressure control strategy in a continuously variable transmission in which a single pressure sensor is used to provide signals for closed loop control of both the clutch pressure regulator and the line pressure regulator and in which open and closed loop controls are operated independently of one another.
2. Description of the Prior Art
There are numerous examples of the operation and construction of continuously variable transmissions (CVTs). For example, U.S. Pat. No. 4,458,318, entitled "Control Arrangement for a Variable Pulley Transmission" and U.S. Pat. No. 4,522,086, entitled "Control System for Continuously Variable Transmission" describe the mechanics of and controls for a CVT system utilizing two adjustable pulleys, each pulley having at least one sheave which is movable and another sheave that is axially fixed with respect to the other. A flexible belt of metal or elastomeric material having a fixed width intercouples the pulleys. The inner faces of the sheaves of the pulleys are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the effective pulley diameter may be adjusted.
The displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. As fluid is forced into or exhausted from the chamber, the pulley diameter is either increased or decreased. Generally the effective diameter of one pulley is moved in one direction as the effective diameter of the other pulley is moved in the other. This enables the ratio between the pulleys to be adjusted.
Developments in CVTs have resulted in improved hydraulic control systems. One such control system is described in U.S. Pat. No. 3,115,049. This patent discloses the regulation of the adjustable sheave of the secondary, or driven, pulley to control the belt tension to protect the sheaves from belt slippage that might result in damage to the belt or pulleys. A different hydraulic circuit, controls the fluid into and out of the primary, or driving, pulley. The change in position of the movable sheave of the primary pulley regulates the transmission ratio. U.S. Pat. No. 4,152,947 also describes control of a CVT. In both systems, the pressure of the fluid applied to hold the belt tension is kept at a relatively high value. An improved control system was subsequently developed to reduce the main line fluid pressure supplied to the secondary sheave chamber in accordance with torque demand. When the torque demand is lower, the tension in the belt necessary to prevent slippage is lower than at high torque demand. This improved system is disclosed in U.S. Pat. No. 4,522,086 entitled "Control System for Continuously Variable Transmission" assigned to the assignee of the present application.
Further work resulted in an improved control system which reduced the line pressure applied to the secondary chamber to a lower operating pressure and also provides a lower control pressure for other portions of a hydraulic control system. This system is described in a copending application Ser. No. 421,198, filed Sept. 22, 1982 now U.S. Pat. No. 4,717,953 and assigned to the assignee of the present application.
Copending application entitled "Control System For Controlling The Line Pressure In A Continuously Variable Transmission", Ser. No. 936,527, filed Dec. 1, 1986, also assigned to the assignee of the present application describes a system for controlling the line pressure in a continuously variable transmission. The system employs a single pressure transducer to realize closed loop control of both the line (secondary) pressure and the clutch pressure. An open loop controller takes over control of the line pressure if the response of the closed loop control is not fast enough to effect desired changes in the line pressure. This prevents belt slippage and consequent damage to the primary and secondary pulleys. In open loop control, an interpolation algorithm is utilized to control the line pressure, thereby reducing the amount of data that must be stored.
The teachings of each of the above referenced patents are incorporated herein by reference. In the control system disclosed in application Ser. No. 936,527, the closed loop control strategy utilizes a signal received from an open loop schedule to regulate the response. This manner of regulation is not as efficient as the utilization of completely closed loop control. It is desirable therefore to provide a control system in which improved efficiency is achieved in closed loop operation.
The stored data required for operation of the previous control system, although reduced because of the interpolation schedules utilized is still relatively large. It is therefore desirable to provide a control system that utilizes a simplified algorithm requiring minimal stored data for fast efficient operation.
Pulse width modulating solenoid valves, such as those often used to control the line pressure in a CVT, tend to be extremely non-linear when operated at high duty cycles of 85-95%. This is due to the response time limitation of the valve. Closed loop operation that results in a duty cycle between these values may cause unstable closed loop control. It is desirable therefore to provide a system that can operate in these conditions without creating unstable operating conditions.
It has also been discovered that problems can arise with the prior control system at the transition between open and closed loop controls. Discontinuities in the output duty cycle are possible that can cause undesirable jumps in the line pressure. It is also desirable therefore to provide a control system that does not produce discontinuities in the output duty cycle at transitions between control modes.
In addition, the prior control system of application Ser. No. 936,527, now U.S. Pat. No. 4,982,822, issued Jan. 8, 1991 for "Control System For Controlling the Line Pressure In A Continuously Variable Transmission, " incorporates elements designed to slow the system's response in closed loop control in order for it not to respond to dynamic effects in the hydraulic circuit. The closed loop control strategy will not therefore respond effectively to relatively fast changes in the desired set point. It has been discovered that the dynamic effects resulting from the distance between the pressure transducer and the secondary sheave can be neglected. It is therefore desirable to provide a control system that operates to respond quickly and efficiently to sudden changes in the desired pressure set point under closed loop control.